M. Lamontagne

Laser generation of convergent acoustic waves for materials inspection

Abstract A laser technique for generating convergent acoustic waves is described. The optically probed Rayleigh wave in the centre of convergence shows an amplification factor of the order of 20 with respect to a collimated surface wave. Applications to the ultrasonic characterization of layered materials and to crack detection are described.

Influence of substrate preparation on the flattening and cooling of plasma-sprayed particles

Impacts of plasma-sprayed molybdenum particles were monitored by detecting thermal radiation emitted by the hot particles when they flatten on the substrate surface. Evolution of the light intensity collected at two different wavelengths was used to obtain information about flattening time, flattening degree, and cooling time of the impinging particles. Variations of these parameters with substrate surface roughness were investigated on glass and molybdenum substrates. The substrate roughness significantly influenced the flattening degree and flattening time of the particles: the smoother the substrate, the larger the surface of the splats and the longer the flattening time. The cooling time, as determined from the decay time of the light signals after impact, was shorter on smooth substrates. In this case, the temperature of the splats was not radially uniform, with a lower cooling rate at the periphery.

Temperature evolution of plasma-sprayed niobium particles impacting on a substrate

Abstract A fast two-color pyrometer is used to monitor the temperature evolution of individual plasma- sprayed niobium particles after their impact on a substrate. The experimental setup permits rejection of thermal events corresponding to in-flight particles intersecting the pyrometer field of view, ensuring that only particles impacting on the substrate surface are examined. Cooling rates as high as 10 8 Ks -1 are measured when particles are sprayed on steel and alumina substrates. An interruption of the rapid cooling process and a transient temperature increase are observed near the niobium melting point. Experimental results are interpreted with the help of a numerical thermal flow model taking into account undercooling and the recalescence effects.

Flattening and solidification of thermally sprayed particles

In this article, molybdenum particles were plasma sprayed on copper, zirconia, and glass substrates. The impact of the molten particles was monitored using a fast two-color optical fiber pyrometer focused on a small spot on the substrate surface. The apparent duration of the flattening process and the cooling speed, both determined from the pyrometer signals, were found to depend on the substrate conditions and to vary with coating thickness. The substrate material and its roughness were also found to influence the texture in the sprayed coatings. Furthermore, a transient thermal flow numerical model was used to compute reliable thermal histories of the impinging particles and the underlying lamellae, the interfacial thermal resistance being determined by comparison of experimental thermograms with computed ones.

Impacting particle temperature monitoring during plasma spray deposition

Instrumentation for monitoring the thermal history of individual plasma sprayed particles as they impact on a substrate is described. A double-wavelength fibre optic temperature sensor is focused on a small spot on the substrate surface to record the cooling rate of particles impacting on this region. Discrimination against in-flight particles intersecting the pyrometer field of view is obtained by a second fibre optic sensor viewing the same spot at an angle and working in coincidence with the first sensor. Typical recorded thermograms are presented and interpreted with reference to a numerical thermal propagation model.

Photographing Impact of Molten Molybdenum Particles in a Plasma Spray

Plasma-sprayed molten molybdenum particles (∼40 µm in diameter) were photographed impinging at high velocity (∼140 m/s) on a glass substrate at room temperature. An optical sensor detected thermal radiation emitted by a droplet as it approached the substrate and activated a time delay unit. After a selected time interval, an Nd:YAG laser was triggered, emitting a 5 ns pulse that provided illumination for a charge-coupled device (CCD) camera to photograph the impacting droplet through a long-range microscope. By varying the delay before pulsing the laser, different stages of droplet deformation were recorded. Impacting droplets spread into a thin circular film that ruptured and broke into small fragments. An optical detector recording thermal radiation from the impacting droplet gave a signal that increased as the droplet spread out, reached a maximum when the liquid film began to rupture, and decreased as portions of the droplet recoiled because of surface tension and then flew out of view of the photodetector.

Influence of the coating thickness on the cooling rates of plasma-sprayed particles impinging on a substrate☆

Abstract Thermal histories of plasma-sprayed molybdenum particles during and after impingement on different substrates were monitored using a fast two-colour pyrometer and the influence of the coating thickness on the lamella cooling rate was investigated. The cooling rate was found to increase with coating thickness owing to the low thermal resistance at the molybdenum-molybdenum lamella interfaces and to the relatively high thermal conductivity of the sprayed molybdenum lamellae. The lower cooling rates observed at the coating-substrate interface were associated with changes in the lamella microstructure, the lamellae within the coating having a columnar structure while this structure was absent in lamellae at the coating-substrate interface.

A hollow waveguide infrared thermometer for polymer temperature measurement during injection moulding

We describe a novel on-line infrared method for remote sensing of the surface and the bulk temperatures of a polymer film during injection moulding. The method may also be applied to other polymer forming processes such as extrusion and blow moulding. The key feature of the new method is the use of a hollow waveguide that is incorporated into the injection mould to transmit the thermal radiation from the target to the sensor. The main characteristic of the hollow waveguide is that it exhibits low transmission loss of the thermal energy in the mid-and far-infrared, and no end reflection. This allows measurement of quite low temperatures, as low as near room temperature. Conventional optical fibre thermometers can neither measure such low temperature ranges nor measure the polymer surface temperature. In this paper, we present the first on-line results of critical tests of the new device. A Husky injection moulding press was used for the experiments. Good correlation was found between the radiometric results and those obtained with a thermal sensor inserted near the polymer mould interface, and with infrared imaging after the polymer part was ejected from the injection mould.

Photographing impact of plasma-sprayed particles on metal substrates

Plasma-sprayed, molten molybdenum particles (∼40 µm diameter) were photographed during impact (with velocity ∼110 m/s) on Inconel surfaces that were maintained at either room temperature or at 400 °C. Some samples were also preheated at 400 °C for 3 h and then air-cooled to room temperature before spraying. A droplet approaching the surface was sensed using a photodetector, and after a known delay, a fast charge-coupled device camera was triggered to capture images of the spreading splat from the substrate front surface. A rapid two-color pyrometer was used to collect the thermal radiation from the impacting particles to follow the evolution of their temperature and size after impact. Molten molybdenum particles impacting the surfaces at room temperature disintegrated and splashed after achieving a maximum diameter >400 µm. Impact on preheated and heated Inconel produced splats with maximum diameters between 200 and 300 µm with less fragmentation. The cooling rate of splats on preheated Inconel was larger than that of splats on nonheated Inconel. Surface analysis showed that preheating Inconel reduced the surface skewness and kurtosis, resulting in improved splat-substrate contact.

Pulsed thermography in the evaluation of an aircraft composite using 3D thermal quadrupoles and mathematical perturbations

This paper is devoted to the characterization of a subsurface flaw within an anisotropic medium during a nondestructive evaluation test using stimulated infrared thermography. A typical illustration is a delamination within a stratified composite material. The originality of the current study consists of providing simple analytical solutions to evaluate the depth and the volume of the flaw in a three-dimensional heat transfer configuration. The volume of the flaw is defined as the product of its lateral extent by its thickness. If the thermal contact resistance of the flaw is known, its lateral extent can be derived from the volume expression. The method proposed here consists of applying first a Laplace transform on the time variable t, then a Fourier transform on the space variables, x and y. The numerical or semi-analytical true solution of integral equations generated by this problem may be very time-consuming, especially in a three-dimensional configuration. We therefore suggest a modelling reduction using the analytical perturbation method written only at its first order. It is however assumed that flaw thermal resistance is small compared to the whole thermal resistance of the material under investigation. The perturbation formalism leads to the construction of approximate analytical solutions that are very convenient for quantitative inversion. The validity of this method has been analysed through a real nondestructive test performed on a calibrated carbon-epoxy laminate of known characteristics.